This invention relates to a monolithic integrated red and blue wavelength laser structure and, more particularly, to a red laser structure which is wafer fused to a blue laser structure.
Independently addressable monolithic dual wavelength light sources, especially arrays that can simultaneously emit two different wavelength light beams from two different laser elements in the monolithic structure are useful in a variety of applications, such as color printing, full color digital film recording, color displays, and other optical recording and storage system applications.
The performance of many devices, such as laser printers and optical memories, can be improved by the incorporation of dual wavelength laser beams. For example, laser printers which use dual beams can have higher printing speeds and/or better spot acuity than printers which use only a single beam. Recent advances in xerography, such as described in commonly assigned Kovacs et al. U.S. Pat. No. 5,347,303 on "Full Color Xerographic Printing System with Dual Wavelength, Single Optical System ROS and Dual Layer Photoreceptor" (which is hereby incorporated by reference), have created quad-level xerography (sometimes referred to as "xerocolography") that enables the printing of three colors (for example, black plus two highlight colors) in a single pass by a single xerographic station.
In these and many applications, closely spaced laser beams of two different wavelengths are desirable.
One way to obtain closely spaced, dual wavelength laser beams is to form two laser emission sites, or laser stripes, on a common substrate. While this enables very closely spaced beams, prior art monolithic laser arrays typically output laser beams at only one wavelength.
Various techniques are known in the prior art for producing two different wavelength laser beams from a monolithic laser array. For example, it is well known that a small amount of wavelength difference can be obtained by varying the drive conditions at each of the two lasing regions. However, the easily achievable but small wavelength difference is insufficient for most applications.
Ideally, for most desired applications, the dual laser elements should emit light of different widely spaced wavelengths. In a preferred monolithic structure, the laser elements would emit light across a widely spaced spectrum from red to blue wavelengths. One problem is that laser sources of different wavelengths require different light emission active layers; i.e. nitride semiconductor layers such as InGaN for blue lasers and phosphide semiconductor layers such as GaInP for red lasers.
One method of achieving these larger wavelength separations is to grow a first set of active layers on a substrate to form a first lasing element which outputs light at one wavelength, and then to etch and regrow a second set of active layers next to the first to form a second lasing element at a second wavelength. However, this method requires separate crystal growths for each lasing element, something which is not easily performed. Furthermore, the phosphide semiconductor structures of red lasers uses a different, non-compatible substrate with the nitride semiconductor structures of blue lasers. Lattice mismatching between semiconductor layers will result in poor or non-existent performance of one or more of the laser structures.
Another technique for obtaining different wavelength laser beams from a monolithic laser array is to use stacked active regions. A stacked active region monolithic array is one in which a plurality of active regions are sandwiched between common cladding layers. Each active region is comprised of a thin volume that is contained within a laser stripe. The laser stripes contains two different active regions that emit laser beams at two different wavelengths.
In a stacked active region monolithic laser array, current flows in series through the stacked active regions. The active region with the lowest bandgap energy will lase, thereby determining the wavelength of the laser beam output from that part of the array. To provide another wavelength output, the previously lowest bandgap energy active region is removed from part of the array and current is sent through the remaining stacked regions.
A major problem with stacked active region monolithic laser arrays is that they have been difficult to fabricate, even with just arsenide and phosphide semiconductor layers. The addition of nitride semiconductor layers makes optical performance nearly impossible and impractical in any real world applications.
It is an object of this invention to provide stacked active region lasers in a monolithic structure capable of outputting closely spaced, multiple wavelength laser beams in the red to blue wavelength spectrum.